The present disclosure relates to microphone apparatuses, and more particularly to microphone apparatuses capable of directional sound pickup (including stereo sound pickup).
It is known that sensitivity of an array microphone, which includes a plurality of microphones arranged at given positions, obtains directivity by applying proper delays, and addition and subtraction to outputs from individual microphones utilizing a difference in acoustic paths from a sound source to the individual microphones.
For example, first and second microphone elements receive sound coming from a given direction. After the second microphone element receives the sound, the first microphone element receives the sound with a delay of Δt. In this case, when an output signal from the first microphone element is delayed by Δt, and then the delayed output signal is added to a signal obtained from the second microphone element, the same signals are superimposed. However, no superimposing effect can be obtained in sounds from other directions. Therefore, with respect to sound from the predetermined direction, the sound pickup sensitivity is improved to enable directional sound pickup.
That is, it is a prerequisite for obtaining directivity to have different acoustic paths from a sound source to microphone elements, which constitute an array microphone. If there are a plurality of acoustic paths from a single sound source to a single microphone element, conditions such as a delay amount and a coefficient for obtaining directivity cannot be properly set. This makes it difficult to obtain excellent directivity.
A first conventional example (see, e.g., Japanese Patent Publication No. 2007-104556, FIG. 1(a)) and a second conventional example (see, e.g., Japanese Patent Publication No. 2007-104582, FIG. 1) suggest mounting a pair of micro-electro mechanical system (MEMS) microphone elements, which prevent an acoustic path from being divided into a plurality of paths to enable stable directional sound pickup.
Structures of microphone apparatuses according to the first and second conventional examples will be described hereinafter with reference to FIGS. 6A and 6B.
FIG. 6A illustrates a cross-sectional structure of the microphone apparatus according to the first conventional example. FIG. 6B illustrates a cross-sectional view of the microphone apparatus according to the second conventional example.
As shown in FIG. 6A, in the microphone apparatus according to the first conventional example, a pair of MEMS microphone elements 111a and 111b, which are arranged in parallel to each other, are mounted on a common mounting substrate 165, and are housed in a capsule 164. The capsule 164 is provided with a partition wall 169 at a center. Each of cavities separated by the partition wall 169 is provided with a sound hole (167a, 167b). In this structure, sound reaching a sound hole provided in one of the microphone elements is not diffracted to reach the other microphone element, and acoustic paths (P1 and P2) can be integrated. This enables directional sound pickup by proper arithmetic processing. Note that FIG. 6A shows a silicon semiconductor substrate 112, a back plate 113, a spacer 114, a through hole 115 for letting air out, an air gap 116, a vibrating plate 133, a signal processor 162, bonding wires 163a-163d, and a sound source 168.
As shown in FIG. 6B, in the microphone apparatus according to the second conventional example, a pair of MEMS microphone elements 211a and 211b, which are arranged in parallel to each other, are mounted on a common mounting substrate 265, and are housed in a capsule 290, as in the above-described first conventional example. The capsule 290 has an acoustic transmissive mesh structure to prevent diffracted sound from leaking into the MEMS microphone elements 211a and 211b. In this structure, acoustic paths (P1 and P2) are integrated. This enables directional sound pickup by proper arithmetic processing. Note that FIG. 6B shows a silicon semiconductor substrate 212, a back plate 213, a spacer 214, a vibrating plate 233, a signal processor 262, bonding wires 263a-263d, and a sound source 268.
However, the above-described first and second conventional examples require the partition wall blocking the MEMS microphone elements from the surrounding space, or the acoustic transmissive mesh structure, respectively, to realize an independent acoustic path. This enables stable directional sound pickup. However, the present inventor has found that there arises a problem that assembly properties and cost efficiency are deteriorated with an increasing number of the components to reduce design flexibility of the substrates.
Furthermore, in order to miniaturize a microphone module, MEMS microphone elements themselves need to be miniaturized. At this time, capacity (the detail will be described later) of respective back air chambers of the MEMS microphone elements is reduced. The present inventor has found that there arises a problem that this leads to a decrease in sensitivity and an S/N ratio of the microphone.
Stereo microphones, which are generally used for video cameras, are required to have higher sensitivity than non-directional microphones used for mobile phones or the like. Therefore, when sensitivity and an S/N ratio of the microphone are to be obtained at given standard values, there have been limitations on miniaturization of MEMS microphone elements.